1. Field of the Invention
The present invention relates to an image forming apparatus for use in a printer, a copier, a facsimile apparatus and the like, and a charging device for a developer usable in such an image forming apparatus.
2. Description of the Related Art
Developing methods which have conventionally been used for an image forming apparatus include a cascade method, a touch-down method and a jumping method.
Various improvements have been made for these methods in order to improve the reproducibility and clarity of an image.
In a conventional structure, a developer carrier for carrying and supplying the developer is located to be opposed to a latent image carrier with a specified distance therefrom, and an appropriate bias voltage is applied between the developer carrier and the latent image carrier. Improved methods using such a system are described in, for example, Japanese Patent Publication Nos. 58-32375, 63-42256, 63-42782, and 64-1013, U.S. Pat. Nos. 4,395,476, 4,473,627, and 4,792,387. The methods described in these publications are referred to as the AC jumping method and have been put into practical use.
Such a conventional AC jumping method will be described with reference to FIG. 8, which illustrates a structure for a developing area and the vicinity thereof of a conventional image forming apparatus. A developer carrier 101 and a latent image carrier 102 are arranged to be opposed to each other. The developer carrier 101 rotates in a direction of arrow A. A blade 107 for forming a developer 100 into a developer layer 103 is provided to be opposed to the developer carrier 101 with a certain distance therefrom. The latent image carrier 102 rotates in a direction of arrow B.
In the conventional AC jumping method, the developer layer 103 is formed on the developer carrier 101 by the blade 107. The developer 100 of the developer layer 103 is charged on the developer carrier 101.
A bias voltage obtained by superimposing an AC bias voltage on a DC bias voltage is applied between the developer carrier 101 and the latent image carrier 102. An alternating electric field is formed between the developer carrier 101 and an area of the latent image carrier 102 opposed to the developer carrier 101 by an AC bias component of the bias voltage. By the alternating electric field, particles of the developer 100 carried on the developer carrier 101 reciprocate between the developer carrier 101 and the latent image carrier 102. While reciprocating, the particles gradually adhere to an image area of the electrostatic latent image on the latent image carrier 102, thereby developing the image.
In order to accurately develop the latent image by the conventional AC jumping method, the particles of the developer 100 on the developer carrier 101 should be attracted accurately to an image area of the latent image carrier 102, the image area having a different potential from that of a non-image area. The particles are attracted to the image area more accurately in the case where the developer layer 103 is more uniformly charged. In order to uniformly charge the developer layer 103, it is indispensable that the developer layer 103 formed on the developer carrier 101 should have a uniform thickness.
Formation of the developer layer 103 having a uniform thickness requires special processing and a precision control mechanism. As a result, problems occur in that the image forming apparatus has a complicated structure and a large size.
The conventional AC jumping method further has the problem of "sleeve ghost" phenomenon. The sleeve ghost phenomenon will be described with reference to FIG. 8.
For example, when the particles of the developer 100 forming the developer layer 103 move to the latent image carrier 102 in order to develop an image 102a, a recess 106 is formed in the developer layer 103. It is impossible to fill the recess 106 by supplying additional particles of the developer 100 since the developer 100 is supplied to the developer carrier 101 as a layer by the blade 107. Accordingly, the additional particles of the developer 100 only turn the recess 106 to a concave portion 104 but do not completely fill the recess 106. When the developer 100 in such a state is supplied to the latent image carrier 102 in order to develop an image 102b, the image 102b has a portion 105 to which the developer 100 does not adhere in a sufficient amount. Such a defect which is generated by an influence from a previous cycle of development is referred to as the sleeve ghost phenomenon.
For charging the developer, a compressive friction charging method and a collision charging method are widely used.
The compressive friboelectric charging method is disclosed in, for example, Japanese Patent Publication No. 59-8831. According to the compressive friboelectric charging method, the developer on the developer carrier is compressed by the blade, and thus is exposed to friction with the developer carrier or the blade. Thus, the developer is charged. When this method is used, the surface of the developer carrier is generally formed of a conductive material in order to apply a charge to the developer.
The collision charging method more actively charges the developer on the developer carrier without using the compressive force of the blade. This method is described in, for example, Japanese Patent Publications Nos. 63-13183, 1-31605, and 1-31606. According to the collision charging method, an AC electric field is formed between the developer carrier and a charging device opposed to the developer carrier. The developer on the developer carrier passes through the area supplied with the electric field while vibrating. Such vibration causes the developer and the developer carrier to collide against each other or causes particles of the developer to collide against one another. In this method also, the surface of the developer carrier is formed of a conductive material in order to apply a charge to the developer.
In either of the above-mentioned two methods, the surface of the developer carrier is formed of a conductive material. Accordingly, the charge of the developer easily leaks through the developer carrier, after being charged by the blade or the charging member until being used for developing the electrostatic latent image. As a result, when the developer is used for development, the developer does not have a sufficient potential or is not uniformly charged. Such inconveniences prevent the image from being developed as specified, and thus lower the developing performance or thwart satisfactory transference on the paper. As the ambient humidity is raised, the charge more easily leaks and thus makes the problem serious.
Further in either of the above-mentioned two methods, the developer is supplied to the developer carrier and used for developing the electrostatic latent image only for a short period of time when the developer carrier and the latent image carrier are opposed to each other. Accordingly, the developing efficiency is too low to correspond to the recent increase in the response speed.